1. Field of the Invention
This invention relates to a disc-shaped magnetic recording medium and a process for producing the same.
2. Related Background Art
In recent years, a flexible disc-shaped magnetic recording medium has been popularly used, such as a flexible disc, as a large volume memory medium for a computer or as a magnetic sheet for a still video camera (video, floppy), etc.
A magnetic recording medium, not limited to a flexible disc or magnetic disc, but also including magnetic tape, performs reading and writing by sliding a magnetic head on a medium. Accordingly, when the magnetic layer of these magnetic recording media is to be formed, magnetic powder is preferably oriented along the sliding direction of the head. In other words, the magnetic powder is preferably oriented in the running direction of a magnetic tape, while in the circumferential direction in a flexible disc or magnetic sheet.
Generally speaking, when a flexible magnetic recording medium is produced by coating a magnetic paint, a base film carrying a magnetic paint thereon passes through two rotating rollers with a gap slightly broader than the base film being formed therebetween. Accordingly, the magnetic powder is oriented in the running direction. This is called mechanical orientation. In general, magnetic powder with greater shape anisotropy, namely, greater axis ratio, is more susceptible to mechanical orientation, and greater angular ratio is obtained as the result of such orientation.
In the case of a magnetic tape, when the product, as coated above by use of a magnetic powder with great axis ratio, is cut into bands, a magnetic tape oriented in the magnetic head running direction is obtained by mechanical orientation. However, when a flexible disc or magnetic sheet is prepared by punching this out into a disc, while the magnetic head slides along the circumference, because of the greater residual magnetization in the coated direction due to mechanical orientation, the envelope wave form does not become constant, thus creating the problem that the output will periodically fluctuate.
For solving this problem, there have been some proposals regarding the method for orientation in a concentric shape, but they have not yet been practically applied. In the presently commercially available floppy disc or video floppy, devices have been made to weaken mechanical orientation in the coating step, or the envelope is made as flat as possible by use of a magnetic powder with small axis ratio. However, since the magnetic powder is not oriented circumferentially, there were problems in that angular ratio became worse, and the residual magnetization was smaller as compared with the oriented magnetic powder, as in a magnetic tape. Accordingly, in such a disc-shaped magnetic recording medium, the track width is required to be broadened for obtaining sufficient output. However, this prevents higher densification.
For example, by taking the example of a metal tape, the axis ratio of the metal magnetic powder can be varied from about 5 to 15, but ordinarily one with an axis ratio of about 10 is used. The angular ratio of the tape with the use of a magnetic powder of such long needles, which is about 0.55 in the case of non-orientation, becomes about 0.6 to 0.65 in the longer direction of the coating line by application of mechanical orientation. Further, as disclosed in Japanese Patent Publication No. 2536/1959, if magnetic orientation is imparted by use of a device in which a sheet coated with a magnetic paint before solidification passes between two plate magnets with the same magnetic poles being opposed to each other, the angular ratio increased to about 0.8 to 0.85. On the other hand, the angular ratio in the circumferential direction of a metal video floppy suppressed in magnetic field orientation is essentially the same as the nonoriented tape, namely about 0.55 to 0.65 and the output is lowered by about 3 dB as compared with the tape imparted with magnetic orientation.
Except for the problems as described above, as an example of a magnetic recording medium circumferentially oriented, there is also a hard disc obtained by spin coating. However, in the case of spin coating, since individual recording media is applied with coating one sheet by one sheet after formation of the support in a shape of a disc, bulk productivity is poor, thus involving the problem of increased cost.
A method for orienting circumferentially with a magnetic field externally applied is disclosed in Japanese Patent Publication No. 23626/1965. According to this method, rotatory magnetic pole surfaces are moved toward each other under the state where the magnetic paint is not solidified, and the magnetic powder is oriented in concentric circles while maintaining the rotational axis lines relatively stationary, and then separated from each other. In Example of this method, the rotatory magnetic field is imparted from one side of the raw fabric, but in the case of a tape, as previously shown in Japanese Patent Publication No. 2536/1959, it is better to provide a rotatory magnetic pole surface having the same magnetic pole on the opposite side as opposed thereto with the raw fabric sandwiched therebetween. A conceptional illustration of such a device is shown in Japanese Laid-open Patent Publication No. 62505/1978.
However, in the orientation method according to the above rotatory magnetic field, the magnetic field orienting force is basically the same at any position on the circumference. On the other hand, as previously mentioned, the mechanical orientation force is strong in the raw fabric running direction. Therefore a difference remains between the orientation in the direction in parallel to the raw fabric running direction and the orientation in the direction perpendicular to the running direction within the raw fabric plane even after application of the magnetic field. Accordingly, although this difference can be smaller by use of a magnetic powder with smaller axis ratio (needle shape ratio), the angular ratio after orientation increases as the shape anisotropy increases. Therefore, for enhancing residual magnetic flux density, and accordingly for generally enhancing recording density, it is better to use a magnetic powder with great axis ratio (needle shape ratio). In that case, there ensues a problem that directional difference becomes further increased in the orientation degree of mechanical orientation.
For eliminating the drawbacks of the prior art as described above, the present Applicant has filed Japanese Patent Applications Nos. 250853/1985 and 258677/1985, and further filed U.S. Pat. Ser. No. 928,420, claiming priority based on the above two Japanese Patent Applications. The gist of these applications resides in producing a magnetic recording medium, comprising coating a nonmagnetic substrate with a magnetic paint and applying a rotating magnetic field for orientation to the magnetic paint in a yet unsolidified state on the substrate; the improvement wherein the magnetic field for orientation is stronger in a direction perpendicular to the moving direction of the substrate than in the moving direction.